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Aerospace Flywheel Development
1) INTRODUCTION
Presently, energy storage on the Space Station and satellites is accomplished using chemical
batteries; most commonly nickel hydrogen or nickel cadmium. A flywheel energy storage system is an
alternative technology that is being considered for future space missions. Flywheels offer the
advantage of a longer lifetime, higher efficiency and a greater depth of discharge than batteries. A
flywheel energy storage system is being considered as a replacement for the traditional
electrochemical battery system in spacecraft electrical power systems. The flywheel system is
expected to improve both the depth of discharge and working life by a factor of 3 compared with its
battery counterpart. Although flywheels have always been used in spacecraft navigation and guidance
systems, their use for energy storage is new. However, the two functions can easily be combined into
a single system. Several advanced technologies must be demonstrated for the flywheel energy
storage system to be a viable option for future space missions. These include high strength composite
materials, highly efficient high speed motor operation and control, and magnetic bearing levitation.
2) COMPONENTS OF FLYWHEEL SYSTEM
The main components of the flywheel energy storage system are the composite rotor,
motor/generator, magnetic bearings, touchdown bearings, and vacuum housing. The flywheel system
is designed for 364 watt-hours of energy storage at 60,000 rpm and uses active magnetic bearings to
provide a long-life, low-loss suspension of the rotating mass. The upper bearing of the unit is a
combination magnetic bearing, providing suspension axially as well as radically. The lower magnetic
bearing suspends the shaft in the radial direction only. At each end of the shaft there is also a
touchdown bearing. This provides a back up bearing system should the magnetic bearings fail during
testing.
The motor/generator unit is located at the lower end of the shaft. It consists of a two-pole
rotor piece with surface mounted samarium cobalt magnets and a carbon fiber retaining wrap. On the
stator side, there are three phase sinusoidally distributed windings in twelve slots. A water jacket
around the stator provides cooling. Field orientation and a combination of mechanical sensor less
techniques are used to control the motor from zero and low speed up to full speed operation. The selfsensing technique is used at zero and low speeds to start the machine, then the control is switched to
a back-EMF based sensor less technique for the normal higher speed operating range of the machine.
A typical system consists of rotor suspended by bearings inside a vacuum chamber to reduce
friction, connected to a combination electric motor/electric generator. On larger systems, the bearings
are magnetic. The rotors are generally made of steel on smaller systems but large systems use hightensile-strength fibers (such as carbon fibers) embedded in epoxy resins, or some other high-strength
composite material. Energy is stored by using an electric motor to increase the speed of the spinning
flywheel. The system releases its energy by using the momentum of the flywheel to power the
motor/generator.
3) FLYWHEEL CONTROL
There are three modes of operation for the flywheel in a spacecraft power system
1) Charge
2) Charge reduction
3) Discharge
In charge mode, the solar array produces enough current to charge the flywheel at its set
point and to provide the required load current. The solar array electronics regulate the DC bus voltage
during charge mode.
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In charge reduction mode, the solar array continues to provide load current but it can not
provide enough current to charge the flywheel at its set point. When this occurs, the DC bus voltage
regulation function is transferred to the flywheel system.
In discharge mode, the flywheel system provides the entire load current and regulates the
DC bus voltage.
3.1) CHARGE, CHARGE REDUCTION AND DISCHARGE CONTROL
In charge mode, the flywheel charges at a constant power, constant DC current rate using
the excess current from the solar array. The charge control algorithm regulates the acceleration of the
flywheel motor so that the DC current is maintained at the commanded set point. There are two
components to the controller: the proportional-integral (PI) and the feed-forward (FF). The feedforward portion uses the DC charging current command and converts it into a motor current
command. The measured DC bus voltage and the estimated rotor speed from the back EMF estimation
algorithm. The PI portion makes up for any inaccuracies in the relationship and guarantees zero
steady state error. Thus fast, accurate performance is achieved with relatively low gains.
In charge reduction and discharge modes, the flywheel motor must decelerate at the
appropriate rate to maintain the DC bus voltage at the commanded value while supplying the
necessary current to the loads Again, there are two components to the controller: the PI portion and a
disturbance decoupling portion (DD). In the decoupling portion, the DC flywheel current is measured
and used as an early indicator to the controller whether there has been an increase or decrease in
load. If there is a sudden increase in load, the capacitor will initially maintain the bus voltage and
there will be an increase in the DC current, Flywheel, to supply the new load. This increase in Flywheel
is measured and used to calculate the corresponding motor current. Thus the motor responds by
decelerating more quickly, even before a drop in the DC bus voltage causes the system is in charge
mode (current regulation) when the solar array provides enough current to meet both the load
demands and the charging current to the flywheel system. Otherwise, the system is in charge
reduction or discharge mode which means the flywheel system is regulating the DC voltage bus.
The transition from current regulation to voltage regulation is accomplished in the following
manner. The solar array regulates the bus voltage to a set point value higher than the flywheel
regulation set point as long as the solar array current is sufficient to provide both the load and the
charging current. Once the solar array current begins to drop off, the DC bus voltage begins to fall and
the flywheel current, Flywheel, also drops. This transition is detected in the controller by comparing
the difference between the actual DC bus voltage and the flywheel set point voltage to the "voltage
transition constant", VTC.Once this difference is less than the VTC, the integrator in the PI portion of
the controller is reset. This reduces the command at point 2 to a value slightly larger than Flywheel.
This value is then compared to the charge current set point. If it is less than, which it will be if the
solar array is not producing enough current, then the system transitions into charge reduction mode
where the DC bus voltage is regulated by the flywheel system. Similarly, as the system moves from
eclipse into sunlight, the solar array will produce more current. When the array produces enough
current to meet the load demand, the command at point 2 in the controller will become positive.
When it exceeds the charge current set point, the integrator in the current regulator portion of the
controller is reset and the system transitions back into charge mode where the flywheel system
regulates the current into the flywheel and the solar array system regulates the DC bus voltage.
The three modes of operation: charge, charge reduction and discharge, were defined based
on a battery energy storage system. Because the flywheel system is intended to replace batteries,
these modes were duplicated in the flywheel system control. However, the flywheel energy storage
system is capable of regulating the DC bus voltage both when charging and discharging, obviating the
need for multiple modes and the transition between them. Designing the flywheel system control to
perform this regulation at all times would result in an overall simpler control strategy, even when
considering the necessary provisions to prevent over-speed or over current operation. This is an area
for future study.
4) AEROSPACE FLYWHEEL CHALLANGES
The primary factor preventing the application of flywheels to long-term energy storage is
loss in the bearings. Any mechanical bearing with contact between the stationary and rotating parts
will have enough loss to render the system uneconomical one solution to the problem is to use a noncontact active magnetic bearing that employs conventional electromagnets. The rotational loss of such
a bearing is 1-10% that of a mechanical bearing under the same operating conditions. The problem,
however, is that the bearing itself consumes power, which is dissipated as heat in the copper
electromagnets, and the bearing and cooling system power consumption must be included in the
calculation of the overall system efficiency. A reasonable magnetic bearing consumes a few watts for
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each kilogram of flywheel weight, depending
and this loss is sufficient to make a
Superconducting magnetic bearings, on the
watts per kg for a 2,000 rpm rotor. This
efficiency of 84%, which is acceptable.
on the structure of the bearing and the control system,
system using copper electromagnets uneconomical.
other hand, have demonstrated losses of 10-2 to 10-3
translates to an overall one-day, "round-trip" system
5) FLYWHEEL ENERGY STORAGE (FES)
It works by accelerating a rotor to a very high speed and maintaining the energy in the
system as inertial energy. Commercially available FES systems are used for small uninterruptible
power systems. The rotors normally operate at 4000 RPM or less and are made of metal. Advanced
flywheels are made of high strength carbon-composite filaments that spin at speeds from 20,000100,000 RPM in a vacuum enclosure. Magnetic bearings are necessary as speeds increase to reduce
the friction present when using conventional mechanical bearings. Quick charging is done in less than
15 minutes. Long lifetimes of most flywheels, plus high energy densities (~ 130 Wh/kg) and large
maximum power outputs are positive attributes. The energy efficiency (ratio of energy out per energy
in) of flywheels can be as high as 90%. Since FES can store and release power quickly, they have
found a niche providing pulsed power.
Figure 5.1: Energy storing of flywheel
6) ADVANTAGES AND DISADVANTAGES
Flywheels are not affected by temperature changes as are chemical batteries, nor do they
suffer from memory effect. Moreover, they are not as limited in the amount of energy they can hold.
They are also less potentially damaging to the environment, being made of largely inert or benign
materials. Another advantage of flywheels is that by a simple measurement of the rotation speed it is
possible to know the exact amount of energy stored. However, use of flywheel accumulators is
currently hampered by the danger of explosive shattering of the massive wheel due to overload.
One of the primary limits to flywheel design is the tensile strength of the material used for
the rotor. Generally speaking, the stronger the disc, the faster it may be spun, and the more energy
the system can store. When the tensile strength of a flywheel is exceeded the flywheel will shatter,
releasing all of its stored energy at once; this is commonly referred to as "flywheel explosion" since
wheel fragments can reach kinetic energy comparable to that of a bullet. Consequently, traditional
flywheel systems require strong containment vessels as a safety precaution, which increases the total
mass of the device. Fortunately, composite materials tend to disintegrate quickly once broken, and so
instead of large chunks of high-velocity shrapnel one simply gets a containment vessel filled with redhot sand (still, many customers of modern flywheel power storage systems prefer to have them
embedded in the ground to halt any material that might escape the containment vessel). Gulia's tape
flywheels did not require a heavy container and reportedly could be rewound and reused after a tape
fracture.
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When used in vehicles, flywheels also act as gyroscopes, since their angular momentum is
typically of a similar order of magnitude as the forces acting on the moving vehicle. This property may
be detrimental to the vehicle's handling characteristics while turning. On the other hand, this property
could be utilized to improve stability in curves. Two externally joined flywheels spinning synchronously
in opposite directions would have a total angular momentum of zero and no gyroscopic effect.
7) APPLICATIONS
In the 1950s flywheel-powered buses, known as gyro buses, were used in Yverdon,
Switzerland, and there is ongoing research to make flywheel systems that are smaller, lighter,
cheaper, and have a greater capacity. It is hoped that flywheel systems can replace conventional
chemical batteries for mobile applications, such as for electric vehicles. Proposed flywheel systems
would eliminate many of the disadvantages of existing battery power systems, such as low capacity,
long charge times, heavy weight, and short usable lifetimes. Flywheel systems have also been used
experimentally in small electric locomotives for shunting or switching.
8) CONCLUSION AND FUTURE WORK
This paper has presented a new algorithm for regulating the charge and discharge modes of
a flywheel energy storage system using a sensor less field orientation control algorithm to provide the
inner loop torque control. The algorithm mimics the operational modes presently found in battery
systems and would allow the flywheel system to replace batteries on future spacecraft. Experimental
and simulation results show the successful control of the flywheel system permanent magnet motor in
all modes of operation. Additionally, transition between modes and DC bus voltage regulation during
step changes in load was also demonstrated. A future application of flywheel technology is to use
flywheels to combine the energy storage and the attitude control functions on a spacecraft. A
minimum of four flywheels would be needed to provide three axes of attitude control plus power
during eclipse
REFERENCES
1.
Anand, D.K., Kirk, J.A., and Bangham, M.L., “Simulation,Design, and
Construction of a flywheel Magnetic Bearing”, ASME paper 86-DET-41, 1986.
2.
Kirk, J. A., Studer, P. A. and Evans, H. E., "Mechanical Capacitor", NASA TND-8185,
March 1976.
3.
SEMINAR TOPIC FROM
4.
Kirk, J. A., " flywheel Energy Storage Part I - Basic
Science, Vol. 19, No. 4, 1977, pp. 223-231.
5.
Kirk, J. A. and Anand, D. K., "Satellite Power Using a Magnetically Suspended
flywheel Stack", Journal of Space Power, Vol. 22, Issue 3&4, March/April 1988
6.
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:: www.edufive.com/seminartopics.html
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Concepts", Int. J. of Mech.